There are many industrial applications where relative positioning of two objects may be sensed by means of magnetic fields. One example is the precise vertical position of an elevator car within an elevator hoistway, shown in FIG. 1. Magnetic strips 12-14 on the wall of the hoistway in the vicinity of each floor landing provide an indication of when the elevator car is within the outer landing zone (12), the inner landing zone (13), and the leveling zone (14), respectively. The magnetic fields of the strips are sensed by corresponding magnetic detectors 15-17. These are shown in FIG. 1 as not being aligned in a row, but rather as being displaced from each other for ease of mounting and to reduce interference; the magnetic strips 12-15 are similarly disposed. The magnetic detectors 15-17 are shown centrally disposed on the magnetic strips 12-14 in the position they have when the elevator is perfectly aligned at the leveling zone of the related floor landing. A set of dotted circles 15a-17a show the position of the three magnetic detectors as an elevator car travels downwardly and first reaches the outer landing zone. Another set of dotted circles 15b-17b show the position of the magnetic detectors 15-17 as the elevator car travels further downwardly and first enters the inner landing zone and first senses the magnetic strip 13. The magnetic strips 12-14 may be mounted together on a sheet of magnetic material 20 which may be disposed on the hoistway wall.
In FIG. 2, the magnetic detector 17 is shown disposed on an elevator car 21 by means of a bracket 22 which is broken away for simplicity. The magnetic strips 13, 14 are also shown disposed on the sheet 20. Not shown in FIG. 2 (for simplicity) is the magnetic detector 16 (which would be similarly mounted on a portion of the bracket 22, or another bracket, in alignment with the magnetic strip 13) and the magnetic strip 12 and its magnetic detector 15 (which would be disposed to the left as seen in FIG. 2). The magnetic detectors 15-17 may typically comprise magnetically actuated microswitches, sometimes called "reed relays", or other suitable sensors As seen in FIG. 2, the flux from the magnetic strip 14 is attracted to the structure of the magnetic detector 17, which helps to define its path, and thus causes rather confined flux lines, indicated by the dotted lines 23. As depicted in FIG. 2, for example, the flux lines are seen to leave the south pole of the magnetic strip 14 and have an upward sense indicated by arrows in FIG. 2, and then return in an outer path, with a downward sense indicated by arrows. The problem is that the flux of an upward sense from the south pole of the magnetic strip 13 has a downward sense in the vicinity of the magnetic detector 17, which attracts the flux lines, and causes them to interfere in an opposite sense, as indicated by solid lines 24. Having flux of opposite sense in the vicinity of the detector 17 reduces its sensitivity. This interference reduces the ability of the detector 16 to sense the presence of a magnetic strip, as is illustrated in FIG. 3.
In FIG. 3, a dash line 27 indicates a point at which a magnetic detector 28 is about to detect the commencement of a magnetic strip 29. But since it is between two magnetic strips 30, 31, which (according to the convention of FIG. 2) cause downward flux, the sensitivity of the detector 28 in detecting the commencement of the magnetic strip 29 is much reduced. As a result of the phenomena illustrated in FIGS. 2 and 3, it is necessary to keep the strips 12-14 separated from each other by a sufficient amount so that the amount of interfering flux illustrated by the lines 24 (FIG. 2) is minimal and does not unduly interfere with the magnetic detector 17 sensing the presence or absence of the flux lines 23 from the magnetic strip 14. Signals indicative of the magnetic indicia sensed by the magnetic detector 17 are provided by electrical conductors 25 to circuitry (not shown), all in the well-known manner.